The use of nuclear quadrupole resonance (NQR) as a means of detecting explosives and other contraband has been recognized for some time—see e.g. T. Hirshfield et al, J. Molec. Struct. 58, 63 (1980); A. N. Garroway et al, Proc. SPIE 2092, 318 (1993); and A. N. Garroway et al, IEEE Trans. on Geoscience and Remote Sensing, 39, pp. 1108-1118 (2001). NQR provides some distinct advantages over other detection methods. NQR requires no external magnet such as required by nuclear magnetic resonance. NQR is sensitive to the compounds of interest, i.e. there is a specificity of the NQR frequencies.
One technique for measuring NQR in a sample is to place the sample within a solenoid coil that surrounds the sample. The coil provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample and results in their producing their characteristic resonance signals. This is the typical apparatus configuration that might be used for scanning mail, baggage or luggage. There is also need for a NQR detector that permits detection of NQR signals from a source outside the detector, e.g. a wand detector, that could be passed over persons or containers as is done with existing metal detectors. Problems associated with such a detector using conventional systems are the decrease in detectability with distance from the detector coil, and the associated equipment needed to operate the system.
A detection system can have one or more coils that both transmit and receive, or it can have separate coils that only transmit and only receive. A transmit, or transmit and receive, coil of an NQR detection system provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample, and results in their producing their characteristic resonance signals that the coil detects. The NQR signals have low intensity and short duration.
The transmit, receive, or transmit and receive, coil preferably has a high quality factor (Q). The transmit, receive, or transmit and receive, coil has typically been a copper coil and therefore has a Q of about 102. It is advantageous to use a transmit, receive, or transmit and receive, coil made of a high temperature superconductor (HTS) rather than copper since the HTS self-resonant coil has a Q of the order of 103−106. The large Q of the HTS self-resonant coil produces large magnetic field strengths during the RF transmit pulse and does so at lower RF power levels. This dramatically reduces the amount of transmitted power required to produce NQR signals for detection, and thereby reduces the size of the RF power supply sufficiently so that it can be run on portable batteries.
The large Q of the HTS self-resonant coil also plays an important role during the receive time. In view of the low intensity NQR signal, it is important to have a signal-to-noise ratio (S/N) as large as possible. As the signal-to-noise (S/N) ratio is proportional to the square root of Q, the use of the HTS self-resonant coil results in an increase in S/N by a factor of 10-100 over that of the copper system. These advantages during both the transmit and the receive times enable a detector configuration that is small and portable. It is important that the transmit, receive, or transmit and receive, coil is tunable so that the resonance frequency of the respective coil can be adjusted after fabrication to the NQR frequency.
An object of the present invention is to provide for the adjustment of the resonance frequency of a high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil to enhance its use in a frequency detection system.